Pressurised aerosols for the administration of medicaments, and indeed for other applications, conventionally contain one or more liquefied chlorofluorocarbons (CFC's) as propellant. Such materials are suitable for use in such applications since they have the right vapour pressures (or can be mixed in the right proportions to achieve a vapour pressure in the right range) and are essentially taste- and odour-free.
In recent years there has been increasing concern about the depletion of the ozone layer in the upper atmosphere. This is believed to be due to the release into the atmosphere of CFC's and has led to a search for alternative agents for use in all applications of CFC's. To this end, aerosols for many applications are now pressurised using pressurised gases such as nitrogen or hydrocarbons. However, such propellants are generally not suitable for use in the administration of inhalation medicaments since they are toxic and/or the pressure within the canister falls each time the device is used which leads to unreproducible dosing.
The use of hydrofluorocarbons as aerosol propellants has also been suggested. However, considerable difficulties have been encountered in finding suspending agents which are soluble in hydrofluoroalkanes and capable of stabilising medicament suspensions.